In preparing my sermon on Isaiah 6, Phil Walters pointed me to this great quote from St. Pope Gregory the Great:

When Isaiah also praising the unity of the Trinity, he says that the voices of the seraphim cried out: Holy, holy, holy. However, lest he should appear to sever the unity of the divine nature, he added, Lord God of hosts. He did not say “Lords” or “Gods” but Lord God. In that way he indicated that the one who was three times called Holy was one God.